A three-phase Variable Reluctance (VR) motor is a stepper motor controlled by three coils. The motor includes a rotor which inherently seeks a favored or stable detent position when current is flowing. In operation, the rotor will resist movement until it reaches the zero torque unstable position, whereupon it will flip to the next stable detent position. By applying current to the coils at appropriate times, however, the commutation of the rotor may be controlled and the motor may be prevented from reaching one or more of its natural detent positions. To date, electric motor designers have been challenged to develop systems and methods to economically and efficiently perform this task.
Consider, for example, the motor torque profile of a typical three-phase Variable Reluctance motor. As shown in FIG. 1, over an angle of forty-five (45) mechanical degrees (zones Z.sub.1 (10), Z.sub.2 (12), Z.sub.3 (14), Z.sub.4 (16), Z.sub.5 (18) and Z.sub.6 (20), each spanning 7.5.degree.), the output torque of the Variable Reluctance motor may be illustrated by three phase-separated, pseudo-sinusoidal waveforms .phi..sub.A (22) (also called "phase A"), .phi..sub.B (24) (also called "phase B"), and .phi..sub.C (26) (also called "phase C"). As known to those skilled in the art, if it were desirable to generate positive torque (to produce a positive angle) over zones Z.sub.1 (10) and Z.sub.2 (12) (0.degree.-15.degree.), it would be desirable to have .phi..sub.A (22) on for the entire displacement, i.e., from point 28 to point 30. If .phi..sub.A (22) is left on past point 30 (past 15.degree.), it still generates positive torque. However, for the amount of current being invested in the motor, the torque declines rapidly. The efficiency of the motor is thus greatly reduced. Point 30 is therefore the optimal spot to turn .phi..sub.B (24) on if it is desired to continue positive torque over zones Z.sub.3 (14) and Z.sub.4 (16) (15.degree.-30.degree.). Likewise, point (32) (30.degree.) is the optimal spot to turn .phi..sub.C (26) on to continue positive torque over zones Z.sub.5 (18) and Z.sub.6 (20) (30.degree.-45.degree.). At point 32 (30.degree.), the cycle repeats itself such that to continue positive torque, it would be desirable to have .phi..sub.A (22) on from 45.degree.-60.degree., .phi..sub.B (24) on from 60.degree.-75.degree., and .phi..sub.C (26) on from 75.degree.-90.degree..
The converse is also true. Thus, as shown in FIG. 1, to produce negative torque, .phi..sub.B (24) should be on in zone Z.sub.1 (10), .phi..sub.C (26) should be on in zones Z.sub.2 (12)-Z.sub.3 (14), .phi..sub.A (22) should be on in zones Z.sub.4 (16)-Z.sub.5 (18), and .phi..sub.B (24) Should be on again in zone Z.sub.6 (20).
While it is known that efficient operation of a three-phase Variable Reluctance motor may be achieved under these conditions, electric motor designers have heretofore had difficulty in designing motors to operate accordingly.
Consequently, a need has developed for a system and method for obtaining and utilizing motor shaft (rotor) position information and corresponding motor torque rankings to efficiently commute a three-phase Variable Reluctance motor. Such a system and method should be particularly suited for use with a typical three-phase Variable Reluctance motor and should not require the use of substantially additional hardware or contacting elements which will add additional expense or wear out.